In a so-called brushless electric motor, the drive current is commutated electronically. For this purpose, the electric motor is usually assigned a converter circuit that is supplied with voltage via an electrical intermediate circuit. The converter circuit passes to the stator coils of the electric motor an electric three-phase current, which generates a stator magnetic field that rotates relative to the stator. The rotor of the electric motor often has one or more permanent magnets that generate a rotor magnetic field that is static relative to the rotor. The interaction of the stator magnetic field with the rotor magnetic field results in a torque that sets the rotor in motion.
The phases of the three-phase current generated by the converter circuit and of the associated stator magnetic field are referred to as motor phases. In the figurative sense, this expression also denotes the stator coils respectively assigned to such a phase with the associated connecting lines. The motor phases are often connected up to one another in a star connection. The converter circuit drives the motor phases depending on the rotor position, which has to be determined metrologically for this purpose. Sensors, such as e.g. Hall sensors, are often provided for determining the position, that is to say determining the angle of rotation, of the rotor. Often, and not least for cost reasons, sensorless angle-of-rotation transmitters are alternatively used for this purpose. The position determination is effected by said angle-of-rotation transmitters by detection of the so-called back electromotive force (also referred to as back-EMF) of the electric motor. This expression denotes the voltage induced in the stator coils by the rotating rotor magnetic field. Motors of this design are referred to as sensorless electric motors.
Conventional sensorless angle-of-rotation transmitters are usually embodied as analog electronic circuits. Such a circuit determines the back-EMF by detection and analysis of all the phase voltages. In order to prevent fluctuations of the phase voltages that arise as a result of switching processes or a pulse width modulation (PWM) from leading to an erroneous position determination, a conventional angle-of-rotation transmitter usually additionally comprises suitable filter circuits that filter out these disturbances. The resulting back-EMF is compared with a comparison voltage in a comparator, wherein the comparator generates a position signal if the back-EMF exceeds a predetermined comparison value.
The position signal is conventionally generated upon a positive zero crossing of the back-EMF (that is to say upon a change in sign of the back-EMF from negative to positive). As an alternative to this, it is customary to operate an electric motor with pretriggering or with posttriggering. The comparison voltage is adapted for this purpose in such a way that the position signal is initiated before or respectively after the zero crossing of the back-EMF.
The back-EMF can only be measured in a motor phase in which the motor current has stopped. In order to measure the back-EMF in a motor phase, therefore, even after the disconnection of this motor phase from the reference potentials of the intermediate circuit, it is necessary to wait during a so-called commutation time period, within which the so-called freewheeling current in the motor phase decays. In this case, freewheeling current denotes the current which is temporarily maintained by the inductive inertia of the motor phase even after the disconnection of the latter. In order to measure the freewheeling current, a conventional angle-of-rotation transmitter is often assigned a separate circuit that initiates or enables the position determination only when the freewheeling current has decayed.
The position signal output by the angle-of-rotation transmitter is usually fed to a microcontroller, which drives the converter circuit, as “trigger signal”.